Fibrillation refers to a condition in which muscle fibrils enter a state of extremely rapid, small-scale contractions that do not coordinate to affect contraction of the muscle as a whole. When this occurs in the left ventricle (LV), the heart chamber responsible for pumping blood into the arterial vasculature, it is serious and rapidly fatal. Atrial fibrillation (AF), fibrillation which occurs in the musculature of the atria and affects as many as 2 million Americans, is less immediately serious and not necessarily fatal. It is still important to treat AF, however, for several reasons. First, AF is associated with a loss of atrio-ventricular synchrony which can be hemodynamically compromising and cause such symptoms as dyspnea, fatigue, vertigo, and angina. AF can also predispose to stroke or cerebral vascular accidents resulting from emboli forming in the left atrium. Although drug therapy, in-hospital cardioversion, and implantable cardioverter/defibrillators are acceptable treatment modalities for AF, a curative approach offers a number of advantages to certain patients, including convenience and greater efficacy.
One such approach is ablation therapy which treats cardiac arrhythmias by destroying myocardial tissue involved in the initiation or maintenance of the tachyarrhythmia. Linear ablation in the right atrium (RA) and/or left atrium (LA) was initially proposed with the purpose of replicating the surgical “maze” procedure (Cox et al., Ann. Thoracic Surg., 56:814 (1993)). However, ablation is most often accomplished by delivering radiofrequency (RF) electrical energy to a catheter electrode that has been placed next to the tissue to be destroyed after identifying ectopic sites or reentrant pathways, e.g., by mapping electrical activation of the atria.
Recent evidence has shown that a high percentage of paroxysms of AF are initiated by trains of rapid discharges originating from the pulmonary veins (PVs) of the LA, and it has been suggested that the autonomic nervous system may play a role in the generation of the ectopic foci in the pulmonary vein, at least in a canine model (Schauerte et al., J. Cardiovascl. Electrophysiol., 12:592 (2001)). Accordingly, catheter techniques have been developed for ablating these sites with RF energy applied from within the PVs. Selective ablation of arrhythmogenic foci, mainly located within the PVs, was reported as an effective treatment for AF (Haissaguerre et al., NEJM, 339:659 (1998)). Another ablation technique involves the production of a circumferential ablation lesion within a PV in order to block the conduction pathway from the vein to the LA, e.g., a circumferential conduction block at the pulmonary vein ostia (Pappone et al., Circ., 102:2619 (2000); Pappone et al., Circ., 109:327 (2004)). However, an effective circumferential lesion must be completely circular. A contiguous line of conduction block may be created from a series of discrete circular RF lesions, or ‘spot welds’ (Schwartzman, PACE, 22:711(1999)). Nevertheless, spot welding is tedious and may be ineffective because the creation of a contiguous circumferential lesion from a series of precisely placed individual RF lesions is difficult to achieve under current imaging limitations of x-ray fluoroscopy as well as other imaging modalities. Moreover, common complications of ablation procedures include thrombus formation, pulmonary venous stenosis, and atrio-esophageal fistula formation. Furthermore, the success rates for pulmonary vein ablation are around 70%, indicating that other mechanisms are involved in AF.
What is needed is an improved method to inhibit AF, e.g., originating from ectopic sites.